Feeder and the like for electric currents of high frequency



1933- E. c. .CORK ET AL 2,138,906

FEEDER AND THE LIKE FOR ELECTRIC CURRENT S OF HIGH FREQUENCY Filed Sept.16, 1936 if 2; j

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. INVENTORS Q L spvga r l gicg col z l AND 3a BYJ L E WSEY ORNY PatentedDec. 6, 1938 UNITED STATES PATENT OFFICE FEEDER AND THE LIKE FORELECTRIC CURRENTS OF HIGH FREQUENCY Application September 16, 1936,Serial No. 100,986 In Great Britain September 1'7, 1935 10 Claims.

This invention relates to aerial systems for electric currents of highfrequency such as are encountered in television and like transmission orreceiving systems.

5 In the specification of British Patent 438,506, there is described afeeder system in which, in one embodiment, a quarter wavelength rejectorcircuit is provided at the end of a concentric feeder which is connectedto a dipole aerial for the purpose of changing an unbalanced concentricline to a balanced aerial system. This balancing efiect is obtained whenequal current is fed into the two portions of the aerial the rejectorcircuit serving to cancel or reduce the current flowing along theoutside of the sheath of the feeder.

It is generally desirable to match the impedance of the aerial to thatof the feeder and hitherto it has been customary to employ a suitableform of transformer for this purpose. In

.many cases the length of the aerial for high frequency transmission orreception is one-half of the operating wavelength, but in some cases, inorder to avoid excessive size of aerial or for other reasons, it may bedesirable to employ an aerial of less length than half of the operatingwavelength. In this case and in the case in which the aerial is of alength greater than one-half of the operating wavelength, the impedanceof the aerial at the point to which the feeder is attached contains areactive component. The mag-' 'nitude of the reactive component alsodepends upon the folding or shaping, if any, of the aerial.

In order, therefore, to obtain efficient transfer of '35 energy betweenthe feeder and aerial and to prevent reflection at the point ofconnection, it is necesary to match the aerial to the feeder whichnecessitates that the system shall be tuned, since the impedance of thefeeder is, in general, almost 40 purely resistive. Since the feederresistance is also in general different in value from the resistivecomponent of the aerial impedance, a transforming device becomesnecessary, as stated above. If the aerial is tuned to a length which isa multiple of the operating wavelength, a

transforming device is still generally required for matching purposes.

In the specification of the aforementioned patent, a transforming deviceis disclosed in which one of the conductors of the feeder is providedfor part of its length equal to a quarter of the operating wavelengthand adjacent the aerial end of the feeder, with a radius greater or lessthan the radius of the main part of the conductor so that such portionhas a characteristic impedancedifferent from that of the main part ofthe feeder, such characteristic impedance being arranged to giveimpedance matching between the feeder and the aerial attached thereto.

It is the chief object of the present invention to provide an improvedhigh frequency aerial system employing a rejector circuit as describedin the specification of the above-numbered patent and in which impedancematching is obtained in a simple, but nevertheless effective manner.

According to the invention, a high frequency aerial system is providedin which a dipole aerial is fed by a concentric feeder comprising aninner conductor surrounded by a sheath and in which a rejector circuitisemployed comprising an auxiliary conductor attached at one end to thesheath, the two portions of the aerial being effectively connected tothe sheath and auxiliary conductor, and wherein, for the purpose ofmatching the impedance of the aerial to that of the feeder, theauxiliary line comprising the sheath and auxiliary conductor is arrangedto function as a transformer. The auxiliary conductor functions in themanner described in the specification of the above numbered patent as arejector, and also functions, in conjunction with the sheath of thefeeder, as a transformer for matching purposes, so that the use of aseparate transforming device, such as the transformer disclosed in theaforesaid specification, or another form of transformer, is avoided. Therequired transforming action may be obtained by connecting a dipoleaerial topoints on the sheath of the feeder and the auxiliary conductorat a suitable distance from the end of the feeder, or by connecting theaerial to the ends of the feeder and auxiliary conductor and byadjusting the length of the auxiliary conductor. Other examples of theinvention will be hereinafter described.

In order that the said invention may be clearly understood and readilycarried into effect, the same will now be more fully described withreference to the accompanying drawing, in which:

Fig. 1 illustrates an aerial system constructed in accordance with oneembodiment of the invention;

Fig. 2 represents the equivalent electrical circuit of the arrangementshown in Fig. 1;

Fig. 3 illustrates a further aerial system in accordance with theinvention;

Figs. 4 and 5 illustrate, respectively, another embodiment of theinvention and the equivalent electrical circuit; and

Fig. 6 illustrates a modification of the system shown in Fig. 4.

Referring now more particularly to Fig. 1 of the accompanying drawing,the aerial system comprises a concentric feeder including a tubularsheath 3 and an inner conductor 4, the feeder at the right hand end ofFig. 1 being adapted to be connected to a short wave wirelesstransmitter or receiver. feeder, an auxiliary conductor 5 is attached tothe sheath 3, the conductor 5 preferably being of the same externalcross section and shape as the sheath 3, but may be otherwiseconstructed as described, for example, in the specification of theaforesaid patent. The auxiliary conductor, as shown, projectsperpendicularly a short distance, for example, one or two inches fromthe sheath 3, and is then bent to lie parallel to the sheath 3, the freeend of the auxiliary conductor being located adjacent the end of thesheath 3. Although the auxiliary conductor is shown mounted parallel tothe feeder, this is not an essential requirement. The free end of theauxiliary conductor 5 is conductively connected by a conductor 6 to theinner conductor of the feeder. The sheath of the feeder. and theauxiliary conductor form an auxiliary transmission line shorted at oneend and connected to the feeder. The arrangement shown in Fig. 1 isillustrated in conjunction with a dipole aerial and the portions 1 and 8of the aerial are connected'to the sheath 3 and the auxiliary conductor5 at points located at a suitable distance from the end of the feeder.

Corresponding to any point of attachment of the aerial, there exists atthe free end of the feeder an impedance which is a function of theimpedance of the aerial and its point of attachment. By suitable choiceof the impedance of the aerial with respect to its point of attachment,or of the point of attachment with respect to the impedance of theaerial, it can be arranged that the effective impedance at the aerialend of the feeder is equal to the characteristic impedance of thefeeder. The impedance looking along the feeder from the end thereofwhich is connected to the transmitter or receiver is then equal tothecharacteristic impedance of the feeder. The action of the auxiliaryconductor in conjunction with the sheath of the feeder is, therefore,that of a transformer.

For the purpose of determining the required dimensions of the systemshown in Fig. 1, reference will now be made to the equivalent electricalcircuit shown in Fig. 2. The aerial impedance in this figure has beenconverted in well known manner to the equivalent circuit of resistanceR. and reactance X in parallel, and is connected across the rejector atthe points BB.

Let Z1 be the characteristic impedance across the sheath 3 and auxiliaryconductor 5, constituting the rejector, considered as part of acontinuous line.

Let Z0 be the characteristic impedance between the inner conductor andthe sheath of the feeder.

Points AA are connected to the feeder, which is a substantially pureresistance of value Z0. To match andtune the aerial it is necessary tomake the impedance at the terminals BB, without the equivalent aerialcircuit connected, equal to a resistance R in parallel with a reactanceX, in order to match the resistance R of the antenna and tune out thereactance +XI of the antenna. This is done by correct choice of theelectrical lengths 91 and 92 of the portions AB and BC of At the other,or aerial end of the .7

the rejector. It is understood, of course, that 9 =21r and where A isthe working wave-length.

.The impedance of the line 91, terminated by Z11 at A is equivalent to aparallel arrangement of resistance R1 and reactance X1, where 2 cos 9+{gj sin 6 1111 2 o FL 22 Z, Similarly the reactance of the shorted line92 is X2=Z1 tan 92 (3) The conditions for tuning and matching arerespectively:

For a given value of equivalent aerial resistance R and some value ofZ1, Equation 6 determines 91, and hence the distance from A at which theaerial is to be connected. Substituting this value of 91 in Equation 2the value of X1 at BB is found. Inserting this and X in Equation 4 X2 isfound and from 3 the value of 92 that is to say the length BC for theassumed value of Z1 is determined.

A consideration of Equation 6 shows that since sin 91 can Vary betweenthe values 0 and 1, R must lie between the limits Z0 and Z1 /Zo.

In the above Z1 has an assumed value and 91 and 92 are determined. Aspecial case arises when in other words when the length AC isone-quarter of the Working wave-length. In this case 91 and Z1 havecertain values. It can be shown that if the aerial has the impedance andis connected across BB at the electrical distance 91 from A, then theimpedance at the open end A is resistive and equal in value to Z0. Hence91 is given by cos 91=r/Zo and X2 cos 9 sin 9 Z is given by Z Fig. 2,Fig. 3 represents a case in which 61:0. Let r+izv be the impedance ofthe aerial.

The equivalent parallel resistance R and reactance X of the aerial aregiven by.

and

1' X: ia-1i The length of the rejector portion is then chosen to have areactance -X. If 6 is the electrical length required, as in Equation r Xtan 9 It is obvious that since R 1, the method of Fig. 3 is onlypossible if o. This is the case that is most likely to occur in practicesince the radiation resistance r of a dipole decreases rapidly with adecrease in length from the approximate value of 70 ohms at half awave-length, and a common value for the characteristic impedance Zo ofthe feeder is 100 ohms.

In the system of Fig. 1 transformations are limited to aerials witheffective parallel resistances between Z and Zl /Z0. In Fig. 4 is shownan alternative scheme in which the, aerial and feeder points ofconnection are interchanged with respect to those of Fig. 1. In thiscase the portions 1 and 8 of the aerial are connected to the free end ofthe sheath 3 and auxiliary conductor and the central conductor 4 istapped by virtue of conductor 5 onto auxiliary conductor 5 at apredetermined distance along its length. By this arrangement aerialshaving an effective parallel resistance outside the above range and anyvalue of reactance can be transformed. This arrangement will alsotransform aerials with resistances within the above range provided thatthe reactance is greater than a value dependent on the resistance.

The equivalent electrical circuit is shown in Fig. 5 and it is possibleto calculate the value of 91 for values of R, X and Z1, i. e., thelength AB is determined. The length of BC is again calculated to cancelthe resultant reactance at BB.

In practice, it is merely necessary to select the tapping point BB sothat the parallel resistance across BB without the central conductor ofthe feeder connected to B is equal to Zn. The length BC is then chosento annul the parallel reactance.

A simpler practical arrangement is shown in Fig. 6. A length of feeder 3a whole number of half wavelengths long, and therefore acting as a 1/ 1transformer, is slidably connected to the conductors 3a and 5 formingthe line of characteristic impedance Z1. The distance AB is adjusteduntil the equivalent parallel resistance measured at D is equal to thefeeder impedance Z0. A shorting member 9 which is the equivalent of theperpendicular portion of the auxiliary conductor 5 of Figs. 1, 3 and 4,and which can be suitably clamped in position, is then adjusted untilthe parallel reactance at D is tuned out. The feeder is thus matched andmay, therefore, be continued for any length.

The term concentric feeder employed herein means a feeder of the kind inwhich one or more conductors are surrounded by an outer conductor in theform of a sheath. The sheath may,

for example, act as one conductor of a two wire feeder or as anelectrostatic shield for the inner conductor or conductors.

What is claimed is:

l. A high frequency aerial system in which a dipole aerial is fed by aconcentric feeder comprising an inner conductor surrounded by a sheathand in which a rejector circuit is em ployed comprising an auxiliaryconductor attached at one end to the sheath at a point intermediate theends of said sheath, the two portions of the aerialbeing effectivelyconnected to the sheath and the auxiliary conductor at predeterminedpoints on the lengths thereof away from their open ends for the purposeof matching the impedance of the aerial to that of the feeder.

2. A high frequency aerial system in which a dipole aerial having acomplex load impedance is fed by a concentric feeder comprising an innerconductor surrounded by a sheath and in which a rejector circuit isemployed comprising an auxiliary conductor attached at one end to thesheath, the two portions of the aerial being effectively connected tothe sheath and auxiliary conductor, said auxiliary. conductor comprisingtwo parallel conducting tubes open at least at one end, the two portionsof the aerial being effectively connected to said tubes at said open endthereof, a. short-circuiting strap connected across said tubes foradjusting the effective length thereof, said inner conductor beingconnected to one of said tubes at a predetermined point along its lengthbetween said strap and said open end, whereby the impedance of saidaerial is matched to the impedance of said feeder.

3. A high frequency aerial system in which a dipole aerial having acomplex load impedance is fed by a concentric feeder comprising an innerconductor surounded by a sheath and in which a rejector circuit isemployed comprising an auxiliary conductor attached at one end to thesheath at a point intermediate the ends of said sheath, the two portionsof the aerial being effectively connected to the sheath and theauxiliary conductor at predetermined points in the lengths thereof awayfrom their open ends, and a connection joining the open end of saidauxiliary conductor and said inner conductor, whereby the impedance ofsaid aerial is matched to that of said feeder.

4. A high frequency aerial system in which a dipole aerial having acomplex load impedance is fed by a concentric feeder comprising an innerconductor surrounded by a sheath and in which a rejector circuit isemployed comprising an auxiliary conductor attached at one end to thesheath at a point intermediate the ends of said sheath, the'two portionsof the aerial being effectively connected to the sheath and auxiliaryconductor at the open ends thereof, said inner conductor being connectedto the auxiliary conductor at a predetermined point along itspredetermined length away from its open end, said auxiliary conductorand said sheath having such lengths and being so constructed as to forma tank circuit and functioning in the manner of a transance of theaerial to that of" the feeder.

5, A high frequency aerial system comprising a dipole aen'al having acomplex load impedance, a concentric feeder having an inner conductorsurrounded by a sheath, and a line for matching the impedance of saidaerial to that of said feeder, said line constituting an auxiliaryconductor and the outer surface of said sheath, said auxiliary conductorbeing coupled at predetermined points in its length both to said innerconductor and to said sheath, the two portions of said aerial beingeffectively coupled both to said sheath and to said auxiliary conductor,whereby the impedance of said aerial is matched to that of said feeder.

6. A high frequency aerial system in which a dipole aerial is fed by aconcentric feeder comprising an inner conductor surrounded by a sheathand in which a rejector circuit is employed comprising an auxiliaryconductor attached at one end to the sheath at a point intermediate theends of said sheath, the two portions of the aerial being efiectivelyconnected to the sheath and the auxiliary conductor at predeterminedpoints on the lengths thereof away from their open ends for the purposeof matching the impedance of the aerial to that of the feeder, thelengths of said sheath and auxiliary conductor to the left and right ofsaid predetermined points being so chosen as to make the aerialconductance equal to the conductance between the predetermined pointsand the aerial susceptance equal and opposite to the susceptance betweenthe predetermined points.

7. A high frequency aerial system in which a dipole aerial is fed by aconcentric feeder comprising an inner conductor surrounded by a sheathand in which a rejector circuit is employed comprising an auxiliaryconductor attached at one end to the sheath at a point intermediate theends of said sheath, the two portions of the aerial being effectivelyconnected to the sheath and the auxiliary conductor at predeterminedpoints on the lengths thereof away from their open ends for the purposeof matching the impedance of the aerial to that of the feeder, saidaerial having an former for the: purpose of matching theimpedjefi'ective parallel resistance in the range between Z and Z1 /Zo,where Z0 is the characteristic im pedance between the inner conductorand sheath of the concentric feeder and Z1 is the value of thecharacteristic impedance across the sheath of the feeder and theauxiliary conductor.

8. Means for matching a balanced complex load impedance to a coaxialline having a certain characteristic impedance, comprising a section oftwo-conductor line, one end of which is open and the other end of whichis short circuited, part of said two-conductor line acting as the outerconductor of a portion of said coaxial line, connections from one ofsaid impedances to the open end of said two-conductor line andconnections from the other impedance to a pair of points on saidtwo-conductor line between which the admittance is equal to theconjugate of the admittance of said last named impedance, the length ofsaid two-conductor line being so chosen as to provide such a point.

9. Means for matching a balanced complex load impedance to a coaxialline having a certain characteristic impedance, comprising a section oftwo-conductor line, one end'of which is open and the other end of whichis short circuited, a part of said two-conductor line havingelectrically the same potential as the outer conductor of said coaxialline, connections from one of said impedances to one end of saidtwo-conductor line and connections from the other impedance to a pair ofpoints on said two-conductor line between which the admittance is equalto the conjugate of said admittance to said last named impedance, thelength of said two-conductor line being so chosen as to provide such apoint.

10. A symmetrical tank circuit devoid of concentrated reactance,connections for energizing it from an unbalanced line, connections fromsaid tank circuit to a balanced load circuit, the tuning of said tankcircuit and the points of connection being so chosen as to match theimpedance of said line to said load.

EDWARD CECIL CORK. JOSEPH LADE PAWSE-Y.

